Resettable overload protective device



Dec. 30, 1958 R. c. CARLSON 2,866,874

RESETTABLE OVERLOAD PROTECTIVE DEVICE Filed Oct. 21, 1957 INVENTOR.

Eaberi LS (hr/5012 aizorneys nnsnrrauan ovnnronn rnorncrrvr: nnvrcn Robert 'C. Carlson, Chicago, 111., assignor to Cook Elec tric Company, Chicago, 111., a corporation of Dela- Ware Application (lctoher 21, 1957, Serial No. 691,229

2 Claims. (Cl. 200-124) This invention relates to electrical overload protective devices of the resettable type and is particularly concerned with such a protective device which can be used as an improved replacement for the so-called grasshopper fuses now widely used in the telephone industry.

Telephone switchboards normally contain a large amount of expensive and delicate apparatus which can be seriously damaged if exposed for long periods to unduly heavy currents. Since this expensive and delicate apparatus is normally connected to external telephone circuits, it is necessary that some type of protective device be used on the incoming lines to insure that the circuits will be opened promptly should an overload occur.

As is well known in the telephone art, unexpected overloads on telephone lines can occur from a variety of causes, including lightning and accidental short circuits with power lines.

For many years the type of overload protector most widely used in switchboard installations has been the socalled grasshopper fuse. Typical large switchboards contain panels carrying dozens or even hundreds of such fuses, mounted in orderly array.

Briefly described, a grasshopper fuse consists of an insulating frame member provided with connector lugs at its opposite ends and having a pair of spring arms extend ing from the connector lugs on opposite sides of the frame. The opposed ends of the spring arms are joined by a fusible wire which passes through an aperture in the frame member and normally holds the two spring arms under torsion. When the current passing through the fusible wire is sufficient to melt it, the wire parts and the spring arms fly apart.

In the usual application, one of the spring arms moves into contacting position with a fixed terminal, closing the circuit to an alarm light which indicates that one of the grasshopper fuses has failed. The other spring arm simply flies upward to an equilibrium position substantially above normal, which makes it conspicuous and easy to detect when the fuse panel is visually inspected.

Thus, when the failure of a fuse is indicated by actuation of the alarm light, a service man can immediately locate on the panel the fuse or fuses which have failed, and they can then be replaced by new fuses after the line has been checked to make certain the overload is no longer present.

The aforementioned procedure, while effective to protect the delicate switchboard equipment against overload damage, is expensive both in terms of material cost and labor cost. Each ruptured fuse must be replaced bya good one, and the replacement operation requires an appreciable amount of time.

The major object of the present invention is to provide a resettable overload protective device which is di rectly interchangeable with the widely used grasshopperfuses and which can be returned to service by a simple mechanical adjustment.

Anotl" r object of the present invention is to provide long or longer than the switchboard equipment it is designed to protect.

While the protection of telephone switchboard apparatus is one of the primary applications of my invention, it should be understood that it can be employed in any application requiring a sensitive protective device in which resettability would be advantageous.

Other objects and advantages of the invention will appear from the following detailed description of a typical embodiment thereof.

In the appended drawing, Figure 1 is a side elevation view of an overload protective device embodying the present invention, it being shown in its normal environment on the fuse terminal board of a telephone switchboard installation. Fig. 2 is an end elevation view of the protective device of Pig. 1. Fig. 3 is a fragmentary view, partly in section, bringing out in detail the manner in which my protective device operates to open the circuit when subjected to overload current. Figs. 4 and 5 are respectively top and bottom views of the Fig. 1 protective device. Fig. 6 is an axial sectional view of the currentsensitive element which forms an important element in the present invention. Fig. 7 is a schematic diagram showing the electrical circuit of my protective device and of its customary environment.

As the drawing shows, a typical overload protective device in accordance with my invention is mounted on a small frame 10, made of fiber or other suitable insulating material. At each end of the frame 10 I provide metallic terminal connectors, marked 11 and 12 respectively. The terminals 11 and 12 are adapted to be received on and clamped by screw connectors such as those indicated at 13 and 14 in Fig. 1, the screw terminals being carried on an insulated supporting panel 15.

Between the terminals 13 and 14, there may be pro- I arm 17 rises from the top of frame ltl as viewed in Fig. 1, I

being an integral extension of the metal lug 11. Spring arm 18 joins the lug 12 on the under side of frame 10. It may either be integrally formed from the same piece of metal used for lug 12 or it may be, as shown in the drawing, a separate piece of spring metal crimped in position by the lug 12 or otherwise securely joined thereto.

Both arm 17 and arm 18 are made of springy material, and their natural positions are opposed-that is, if not restrained arm 17 will occupy a position well above the frame 10, as indicated in dotted lines in Fig. 1, While under like conditions the arm 18 will occupy a position substantially below the frame 10, as also indicated by dotted lines in Fig. 1.

Spring arm 18 has an upward extension 18a which rises alongside the frame 10 to a position somewhat thereabove, terminating in an inwardly projecting pawl 18b. Spring arm 18 also has a downwardly projecting contact member 18c located at the end of the main portion of arm 18 and disposed directly beneath the, frame 10.

The upper spring arm 17 is curved back, upon itself at its outer end and is in its end region threadedly apertured at 17a. Screwed tightly into the aperture 17a is the threaded end 21a of a shaft 21, shaft 21' being an important part of a current-sensitive element generally designated on the drawing with the reference numeral 25. I shall presently describe this sensitive element 25in detail.

The shaft 21 is provided, just inward of the threaded portion 21a, with a land 21b, which limits the maximum distance that shaft 21 can be screwed into the aperture 17a of arm 17. The portion of the shaft 21 to the right of land 21b is smooth and unthreaded.

I shall now describe in some detail the structure of the current-sensitive unit 25. While it is not per se my invention, it forms an important element in my inventive combination. 7

Overlying the shaft 21 is a metal sleeve 22, extending along nearly the entire length of shaft 21 beyond the land 21b. The end of shaft 2'1 opposite land 21b is provided with a washer 23, held in place by deforming the end of shaft 21 or by any other desiredmeans'. Washer 23 prevents any substantial degree of axial movement between the sleeve 22 and the shaft 21 The shaft 21 and the sleeve 22 are soldered together along a part of their respective lengths, as indicated on-the drawing by the numeral 24; The'particular type of solder used for this purpose will depend upon the application; generally speaking, however, it is desirable to use an approximately eutectic mixture of tin and lead so as to provide a solder of low melting temperature.

Surrounding the sleeve 22 is a coil of wire 26, the function of which is to generate-heat respon'sively to the flow of excessive current through it. To this end,- the coil 26 should be wound of a wire having resistance properties appropriateto the application. As will presently be explained, circuit-breaking action takes placewith my invention when the solder 24 has been heated by the coil 26' sufliciently to soften. Coil 26 should be designed,

both as to number of turns and size and-type of wire, with this as the primary standard, the designer keeping in mind, of course, the minimum overload current at-which he wishes the protective device to operate.

One terminal of the coil 26 is electrically connected to the sleeve 22. The other terminal .of coil 26 is connected electrically to a barrel element 27, made of metal and supported by insulating spacers 28 made of fiber or other suitable material. Barrel element 27 is tightly crimped around the spacers 28 so that the barrel 27 is held against rotation relative to the sleeve 22, even though 'the two parts are electrically insulated from one another, except by the circuit through coil 26.

Disposed circumferentially around the outer surface of barrel 27 are a train of ratchet teeth 29; These teeth are designed for cooperation with the pawl 18b.

When my protective device is' set-for normal operation, the current-sensitive element 25, being tightly screwed into the aperture 17a, is pres'seddownward to the position at which one of the ratchet teeth 29 engages and is held by the pawl 18b. Under these conditions, both the springarm 17 and the spring arm 18 are subjected to torsion, the par-ts occupying the positions shown in solid lines in Fig. 1. device under the same Fig. 2

operating conditions is shown in I- s'hall now describe the operation ofmy overloadarm'17, shaft'21, sleeve 22, coil 26, barrel27, pawl 18b, andspr'ing arm 18. Y

So long as the cur-rent conditions in the external circuit are normal, the current flowing throughcoil 26 An end view of the protective 4 is too low to produce suificient heat to soften the solder 24.

Should something occur on the external circuit, however, to cause a much heavier-than-normal current to flow through the protective device, the resulting heat generated in coil 26 by the excessive current will soften solder 24. The instant this happens, the sleeve 22 will slip on shaft 21, and the tooth 29 which is engaging pawl 18h willslip through a suflicient arc to release the pawl. Thereupon the spring arm 17 will rise into its unrestrained position, indicated by dotted lines in Fig. 1. At the same time, the spring arm 18 will drop to its normal unrestrained position, at which the contact 18c at the end of arm 18 will bear against the fixed contact 16 on the terminal board, as indicated in dotted lines in Figs. 1 and 7 The instant pawl 18b is released by the slippage of the ratchet tooth 29, the circuit is broken. As a result, current ceases to flow through the coil 26, the solder 24 promptly cools, and the sleeve 22 and shaft 21 are thus again held securely against relative rotation.

Thus the current-sensitive unit 25 is again ready to function as a protective device, once the overload has been removed from the external circuit.

When the contact terminal 18c engages the fixed contact 16, the warning'lamp 30 will promptly light up, it being energized via terminal 14 from any suitable source of power, in the manner which has been long conventional with grasshopper-fuse installations.

When the warning lamplights up, service personnel will be warned that one of the protective circuits has experienced an overload and has tripped. Visual inspection of the panel 15 will at once reveal which of the protective devices has been overloaded, since the arm 17 will under those circumstances be holding the element 2 5 prominently exposed. This will immediately identify the affected circuit, willenable service personnel to insure that the overload has been removed, and will thus permit the-circuit to be made ready for further normal use.

When this has been accomplished, the service attendant simply presses the element 25 toward the frame 10 until pawl 18b is brought into engagement with another one of the teeth 29: By this act, the normal circuit through the protective device is restored, and it will continue to operate normally until and unless it is tripped by another overload.

Since an overload does not in any way damage the current-sensitive element 25, but merely rotates the sleeve 22- through a few degrees of are on the shaft 21, a circuit protective device embodying my invention can be used indefinitely, even though it may be subjected to numerous overloads.

While I have in this specification described in considerable detail a typical embodiment of my invention, it should be understood that the description is intended to be illustrative rather than limiting. The scope of myinvention should be determined primarily by reference to the appended claims.

I claim:

1. A resettable overload protective device for electric circuits comprising a holder of generally oblong, uniplanar shape havinga conducting lug at each end adapted to be received on a fuse board having a pair of spaced screw contacts and a fixed grounding contactdisposed therebetween, an electrically conducting spring arm connected to one of said lugs, said spring-'arm.when unrestrained occupying a position substantially spaced away from said holder, at second electrically conducting spring arm connected to the other said lug, said other arm having a pawl thereon, a current-sensitive element having a shaft part and a barrel part coaxially disposed, said shaft part being rigidly secured to said first arm and said barrel part being insulated from said shaft part and normally held against rotational relative movement with respect thereto, electrically conducting means connected in circuit between said shaft part and said barrel part and situated therebetween operative to free the barrel and shaft parts for relative rotational movement when the current therethrough substantially exceeds a predetermined value, said barrel part being provided externally with a train of pawl-engaging teeth circumferentially disposed thereon, said barrel part being positioned with one of said teeth in engagement with said pawl when said protective device is set for ordinary operation, said first spring arm being under those conditions displaced from its normal position and subjected to strain, rotational movement of said barrel part responsively to said excess current being operative to free said barrel part and said pawl, enabling said first spring arm to relieve said strain and to assume its said normal position, both said first and second spring arms being displaced from their normal position when said pawl is in engagement with one of said teeth, said second arm being provided with an electrical contact member adapted to engage said grounding contact when said second arm is free to seek its normal position.

2. The structure of claim 1 wherein said first spring arm is normally disposed on the upper side of said holder and said second spring arm is mounted on the lower side of said holder, said second spring arm having a projection extending into the region above said holder, said pawl References Cited in the file of this patent UNITED STATES PATENTS 776,218 Cook Nov. 29, 1904 785,407 Cook Mar. 21, 1905 792,315 Cook June 13, 1905 800,824 Rolfe Oct. 3, 1905 817,160 Cook Apr. 10, 1906 2,207,960 Weber July 16, 1940 FOREIGN PATENTS 185,492 Germany Feb. 3, 1906 703,688 Germany Mar. 14, 1941 

